A typical insulated electric power cable generally comprises one or more high potential conductors in a cable core that is surrounded by several layers of polymeric materials including a first semiconducting shield layer (conductor or strand shield), an insulating layer, a second semiconducting shield layer (insulation shield), a metallic wire or tape shield used as the ground phase, and a protective jacket. Additional layers within this construction such as moisture impervious materials, are often incorporated.
Polymeric semiconducting shields have been utilized in multilayered power cable construction for many decades. Generally, they are used to fabricate solid dielectric power cables rated for voltages greater than 1 kiloVolt. These shields are used to provide layers of intermediate resistivity between the high potential conductor and the primary insulation, and between the primary insulation and the ground or neutral potential. The volume resistivity of these semiconducting materials is typically in the range of 10.sup.-1 to 10.sup.8 ohm-centimeters when measured on a completed power cable construction using the methods described in ICEA (Insulated Cables Engineers Association) specification number S-66-524 (1982), section 6.12, or IEC (International Electrotechnical Commission) specification number contain a polyolefin, conductive carbon black, an antioxidant, and other conventional ingredients such as organic peroxide crosslinking agents, process aids, and performance additives. These compositions are usually prepared in granular or pellet form. Polyolefin formulations such as these are disclosed in U.S. Pat. Nos. 4,286,023; 4,612,139; and 5,556,697; and European Patent 420 271.
The primary purpose of the semiconducting stress control shield between the conductor and insulation within an electrical power cable construction is to ensure the long term viability of the primary solid insulation. The use of extruded semiconducting shields essentially eliminates partial discharge within the cable construction at the interface of conductive and dielectric layers. Longer cable life is also realized through improvement of the conductor shield interfacial smoothness, which then minimizes any localized electrical stress concentration. Polymeric conductor shields with improved smoothness have been demonstrated to extend the cable life through accelerated testing (Burns, Eichhorn, and Reid, IEEE Electrical Insulation Magazine, Vol 8, No. 5, 1992).
A common means to achieve a smooth conductor shield interface is to prepare the semiconducting formulation with acetylene carbon black. Due to the nature of the acetylene carbon black, relative to furnace process carbon black, fewer surface defects are observed on an extruded surface. The primary disadvantage of acetylene black is cost as it is often much more expensive and difficult to manufacture than conventional furnace black.
Furnace carbon blacks are generally easier to use for the manufacture of a semiconducting conductor shield materials. Several commercial carbon black grades described in ASTM D 1765-98b have been used to prepare polymeric semiconductive materials for over forty years, such as N351, N293, N294 (now obsolete), N550, and N472 (now obsolete). However, many of these furnace carbon blacks exhibit poor surface smoothness on the final semiconducting polymeric product.
It is well known that the surface smoothness of an extruded article can be improved by using carbon blacks with larger diameter particles, or, rather, lower surface area. This effect is demonstrated in European Patent 420 271 and Japanese Kokai No. 60-112204.
At the same time, the resistivity of a carbon black based material is related to particle size. That is, larger carbon black particles result in higher, or poorer, resistivity. Hence the two requirements stated here are contradictory requirements. As particle size is increased in order to improve the surface smoothness, the resistivity of the material is increased to an undesirable level.
For a polymeric semiconducting material to be useful for application in an insulated power cable design, the resistivity should be below a fixed value for the product to function correctly. This value is generally stated in power cable specifications, such as IEC specification number 60502 (1996) and AEIC (Association of Edison Illuminating Companies) specification number CS5 (1994), as 10.sup.5 ohm-centimeters maximum at the temperature rating of the cable, generally 90 degrees C for crosslinked polyethylene cable.
Industry is constantly seeking semiconducting formulations, which meet the above requirements and exhibit improved surface smoothness relative to existing commercial carbon black based materials at lower cost.